fix: enforce additionalProperties:false in generated models

[FanouZeng-TT]

Description

Three object schemas in the pinned spec declare additionalProperties: false with named properties:

• types/error_response.json (root ErrorResponse)
• types/merchant_fulfillment_config.json (allows_multi_destination)
• types/business_fulfillment_config.json (allows_multi_destination)

Generation runs with --extra-fields=allow, so every generated model was ConfigDict(extra="allow"). Unknown keys were silently retained in model_extra instead of being rejected:

AllowsMultiDestination.model_validate({"shipping": True, "bogus": "x"})  # accepted today

Fix: the post-processor scans the preprocessed schemas for objects with additionalProperties: false and non-empty named properties, maps each to its generated class (root title or the nested property path, e.g. allows_multi_destination -> AllowsMultiDestination), and flips that class's model_config to extra="forbid". Sibling classes in the same module keep extra="allow".

Category (Required)

• Core Protocol: Changes to the core UCP specification. (Requires Technical Council approval)
• Governance/Contributing: Changes to governance or contributing processes. (Requires Governance Council approval)
• Capability: Changes to a UCP capability. (Requires Maintainer approval)
• Documentation: Documentation-only changes. (Requires Maintainer approval)
• Infrastructure: CI/CD or repository infrastructure changes. (Requires DevOps Maintainer approval)
• Maintenance: Dependency or repository maintenance. (Requires DevOps Maintainer approval)
• SDK: Language-specific SDK updates and releases. (Requires DevOps Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool. (Requires Maintainer approval)
• Community Health (.github): Changes to organization community health files. (Requires DevOps Maintainer approval)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide and Code of Conduct.
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove my fix is effective.
• New and existing unit tests pass locally with my changes.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk (SDK 0.4.x targets UCP release/2026-04-08).

Screenshots / Logs (if applicable)

• ./generate_models.sh 2026-04-08: postprocess reports extra=forbid on 'AllowsMultiDestination' and extra=forbid on 'ErrorResponse'
• python -m unittest discover -s tests -p 'test_*.py': 67 passed
• pre-commit run --all-files: passed
• git diff --check: passed